Vault credential background refresh now supported for mcp_oauth authentication

AI Impact Summary

The update enables automatic background renewal of Vault credentials issued via mcp_oauth, removing the need for manual refreshes in long-running Vault-authenticated workflows. This improves credential reliability for services that repeatedly fetch secrets from HashiCorp Vault, reducing token expiry-related failures. Operators should verify renewal windows and monitoring are aligned with existing rotation policies to avoid silent refresh gaps and ensure visibility into credential lifecycles.